Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine's crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of an engine's camshaft. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type cam phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type cam phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, as required to meet current or anticipated engine operating conditions. In prior art cam phasers, the rotational range of phaser authority is typically about 50 degrees of camshaft rotation; that is, from a piston top-dead-center (TDC) position, the valve timing may be advanced to a maximum of about −40 degrees and retarded to a maximum of about +10 degrees. The phase authority of a vane-type cam phaser is inherently limited by the vanes of the rotor which will contact the lobes of the stator. Limiting the phase authority is important to prevent over-advancing and over-retarding which may, for example, result in undesired engine operation and engine damage due to interference of the engine valves and pistons.
While vane-type cam phasers are effective and relatively inexpensive, they do suffer from drawbacks. First, at low engine speeds, oil pressure tends to be low, and sometimes unacceptable. Therefore, the response of a vane-type cam phaser may be slow at low engine speeds. Second, at low environmental temperatures, and especially at engine start-up, engine oil displays a relatively high viscosity and is more difficult to pump, therefore making it more difficult to quickly supply engine oil to the vane-type cam phaser. Third, using engine oil to drive the vane-type cam phaser is parasitic on the engine oil system and can lead to requirement of a larger oil pump. Fourth, for fast actuation, a larger engine oil pump may be necessary, resulting in additional fuel consumption by the engine. Lastly, the total amount of phase authority provided by vane-type cam phasers is limited by the amount of space between adjacent vanes and lobes. A greater amount of phase authority may be desired than is capable of being provided between adjacent vanes and lobes. For at least these reasons, the automotive industry is developing electrically driven cam phasers.
One type of electrically driven cam phaser being developed is shown in U.S. patent application Ser. No. 12/536,575; U.S. patent application Ser. No. 12/825,806; U.S. Provisional Patent Application Ser. No. 61/253,982; and U.S. Provisional Patent Application Ser. No. 61/333,775; which are commonly owned by Applicant and incorporated herein by reference in their entirety. The electrically driven cam phaser is an electric variable cam phaser (eVCP) which comprises a flat harmonic drive unit having a circular spline and a dynamic spline linked by a common flexspline within the circular and dynamic splines, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of an engine camshaft or an engine crankshaft driven rotationally and fixed to a housing, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an electric motor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. Unlike vane-type cam phasers in which the phase authority is inherently limited by interaction of the rotor and stator, there is no inherent limitation of the phase authority of the eVCP. The eVCP is also capable of provide a phase authority of 100 degrees or even more if desired for a particular engine application.
U.S. Pat. No. 7,421,990 discloses an eVCP comprising a harmonic drive unit. The eVCP of this example uses a phase range limiter that is bolted to the camshaft. The phase range limiter protrudes through an arcuate slot formed in a sprocket wheel. The two ends of the arcuate slot constrain movement of the phase range limiter and thereby limit phase authority of the eVCP. However, this arrangement for limiting the phase authority of the eVCP requires additional components and assembly time. Additionally, since the phase range limiter is external to the eVCP, it may be susceptible to damage which would affect the phase authority of the eVCP.
What is needed is an eVCP with means for limiting the phase authority of the eVCP. What is also needed is a robust means for limiting the phase authority of the eVCP which does not require the addition of components to the eVCP.